This invention relates to a fuel supply control method for electronically controlling the quantity of fuel being injected into an internal combustion engine, and more particularly to a fuel supply control method of this kind, which is adapted to control to desired values the quantity of fuel being injected into the engine in synchronism with a control signal having a certain constant pulse repetition period and generated independently of rotation of the engine, so as to improve the startability of the engine as well as the driveability at acceleration.
A fuel supply control system adapted for use with an internal combustion engine, particularly a gasoline engine has been proposed e.g. by Japanese Patent Provisional Publication (Kokai) No. 57-137633, which is adapted to determine the valve opening period of a fuel injection device for control of the fuel injection quantity, i.e. the air/fuel ratio of an air/fuel mixture being supplied to the engine, by first determining a basic value of the valve opening period as a function of engine rpm and intake pipe absolute pressure and then adding to and/or multiplying same by constants and/or coefficients being functions of engine rpm, intake pipe absolute pressure, engine cooling water temperature, throttle valve opening, exhaust gas ingredient concentration (oxygen concentration), etc., by electronic computing means.
According to this proposed fuel supply control system, the calculations of the valve opening period, i.e. fuel injection quantity and the operation of the fuel injection device are executed in synchronism with a top-dead-center (TDC) signal which is generated synchronously with rotation of the engine. When the magnitude of acceleration required for the engine to perform exceeds a predetermined value, such as at sudden acceleration, in addition to accelerating fuel quantity increase according to the above control synchronous with the TDC signal, another accelerating fuel quantity increase is applied at the same time, which is executed in synchronism with a control signal having a certain constant pulse repetition period and being independent of the TDC signal (asynchronous accelerating fuel quantity increase control), so as to make up for a shortage in the increasing fuel amount obtained by the TDC signal-synchronized control at acceleration of the engine, thereby enhancing the output characteristic of the engine.
However, in such fuel supply control system, when the accelerator pedal is repeatedly stepped on many times immediately before or upon the start of the engine, an increase in the fuel quantity being supplied to the engine takes place due to the action of the system under the above asynchronous accelerating fuel quantity increase control, each time the accelerator pedal is stepped on, resulting in the supply of an excessive amount of fuel or an overrich mixture to the engine, thereby impeding smooth starting of the engine.
In order to eliminate the above disadvantage, it has been proposed to prohibit such accelerating fuel quantity increase while the rotational speed of the engine is below a cranking speed of the engine (Japanese Patent Publication No. 54-27490), and it has also been proposed to employ a circuit for preventing re-triggerring of a generator circuit for generating pulses of a control signal for increasing the fuel quantity at acceleration of the engine under a certain condition even when the engine is determined to be in an accelerating condition.
However, even with these proposed methods, it is difficult to achieve both satisfactory startability and satisfactory driveability at acceleration of the engine. That is, according to the former proposed method, no fuel increase is made at the start of the engine, which is disadvantageous in obtaining improved startability of the engine. Rather, the supply of a suitable amount of fuel to the engine will improve the startability of the engine at the start of the engine. On the other hand, according to the latter proposed method, if the re-triggering of the pulse generator circuit is prevented between generations of adjacent pulses of the TDC signal while simultaneously only one pulse of the fuel quantity increasing control signal, which is asychronous with the TDC signal, is generated between generations of adjacent pulses of the TDC signal, this will result in unsatisfactory driveability of the engine when the engine is accelerating in a region where the engine rotational speed is higher than the cranking speed, because, while such generation of only one pulse of the fuel quantity increasing signal asynchronous with the TDC signal will be sufficient to obtain satisfactory driveability when the engine is accelerating in a low speed region such as the cranking speed. Rather, generation of a plurality of such TDC signal-asynchronous fuel quantity increasing pulses to cause repeated fuel injections when the engine is accelerating in the above high speed region will result in that an air/fuel mixture can be supplied to the engine cylinders, with the fuel and the air uniformly mixed together, obtaining perfect combustion within the engine cylinders and consequently improving the driveability of the engine.